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A Sodium Laser Guide Star Coupling Efficiency Measurement

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A Sodium Laser Guide Star Coupling Efficiency Measurement Research in Astronomy and Astrophysics manuscript no. (LATEX: content.tex; printed on September 21, 2021; 19:04) A Sodium laser guide star coupling efficiency measurement method Feng Lu1, Zhi-Xia Shen1, Suijian Xue1, Yang-Peng Li1, Kai Jin2, Angel Otarola4, Yong Bo3, Jun-Wei Zuo3, Qi Bian3, Kai Wei2, Jing-Yao Hu1 1 Key Laboratoryb of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China; [email protected] 2 The Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China 3 Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China 4 Thirty Meter Telescope Corporation, Pasadena, California, United States Received 2016; accepted 2016 Abstract Large telescope’s adaptive optics (AO) system requires one or several bright arti- ficial laser guide stars to improve its sky coverage. The recent advent of high power sodium laser is perfect for such application. However, besides the output power, other parameters of the laser also have significant impact on the brightness of the generated sodium laser guide star mostly in non-linear relationships. When tuning and optimizing these parameters it is necessary to tune based on a laser guide star generation performance metric. Although re- turn photon flux is widely used, variability of atmosphere and sodium layer make it difficult to compare from site to site even within short time period for the same site. A new metric, coupling efficiency is adopted in our field tests. In this paper, we will introduce our method for measuring the coupling efficiency of a 20W class pulse sodium laser for AO application during field tests that were conducted during 2013-2015. arXiv:1605.06205v1 [astro-ph.IM] 20 May 2016 Key words: instrumentation: adaptive optics; methods: observational; atmospheric effects 1 INTRODUCTION Adaptive optics is one of the latest technology that significantly improved the performance of large ground- based astronomical telescope in terms of image sharpness and sensitivity. The functioning of the system relies strongly on the detection performance for the turbulence induced abberated wavefront which coming from a bright on-sky reference source that should be within isotropic angle from the observed target (Hardy (1998)). The sky coverage of such bright stars is reported to be less than 1% in the near infrared band (B.L. Ellerbroek(1998)), which severely limits the application of AO system. The introduction of artificial laser guide star technology alleviates this problem. By projecting a suitable format laser in close direction ∗ Supported by the National Natural Science Foundation of China, Granting Number 11303056. 2 L. Feng et al. of the observed target, one could generate an artificial guide star in the sky that will lowers the requirement of the brightness of Natural Guide Star (NGS), thus improves the sky coverage of the AO system. There are two methods in generating laser guide star, one is taking advantage of Rayleigh backscatter induced by light scattered by large molecules and dust in the lower atmosphere (0∼20km), another is by exciting the sodium atoms in high atmosphere (90∼110km) with sodium laser and using the resonant fluorescence of sodium atom as the reference signal. Because the sodium laser guide star has a higher altitude which is beneficial for sensing a larger volume of turbulence than Rayleigh laser guide star, it is preferable for laser guide star generation. The brightness of the sodium laser guide star has direct impact on the wavefront detection performance of adaptive optics system. Pulsed laser when combined with range-gating technique, could avoid the frat- ricide effect which is caused by the Rayleigh backscatter in lower atmosphere. The first generation sodium pulse lasers have output power merely of a few-watts level. However, it is soon found out that by fur- ther increasing the output power or reducing the pulse width, the returned flux would be easily saturated. Theoretical modeling (Holzlohner¨ et al.(2010a), Holzl ohner¨ et al.(2010b), Rampy et al.(2012a)) shows that it is necessary to tune laser’s temporal/spectrum behavior as well as other characteristic parameters to take advantange of the physics of sodium atom to further increase the returned flux. Optimum laser’s parameter set has to be determined with on-sky test based on certain metric which should be able to reflect the absolute performance of the laser during laser guide star generation. In earlier papers, the metric used was often reported to be returned photon flux measured by differential photometry with Johnson V band filter. This metric is fine for tuning if the duration of the test is short comparing to the variability of sodium abundance. However, the sodium abundance in the atmosphere is possible to change from 2 × 1013 to more than 10 × 1013 atoms/m2 in one night (Pfrommer et al.(2009)), and it could be even higher in short term due to sporadic pocket of sodium concentration in the atompshere. A new metric was used in Holzloehner’s simulation paper (Holzlohner¨ et al.(2010a)), coupling efficiency of the sodium laser, which was formerly used in Lidar equation. We repeat the Lidar equation hereby in equation1. The coupling efficiency of the 2 laser sce is on the left side. On the right side, the returned flux in unit receiver area F (unit photons/s/m ) is X normalized with laser power in mesosphere P (Ta) , the sodium column abundance CNa and considering the airmass X and the height of the sodium layer L. FL2 sce = 2X (1) P (Ta) CNaX The coupling efficiency thus has the advantage that it is invariant from changes in sodium abundance, sodium layer height, atmospheric transparency, laser power variations if all parameters in the equation could be measured synchronously at the same location. The complexity and cost of the measurement system hin- ders the popularity of this metric. However, because it directly reflects the absolute performance of the laser in generating laser guide star, it is the most helpful metric for optimizing sodium laser’s internal parameters in the field or comparing with numerical simulations. Since 2011, we have developed and improved out measurement method for this parameter and used this method throughout our prototype lasers’ field tests. In this paper, we will introduce our measurement method and present a comparison between one of our latest field test results using this method and simulation result. A Sodium laser guide star coupling efficiency measurement method 3 Parameter name Measurement equipment Description photometry telescope Planewave 12.5 inch Corrected returned flux of laser guide star (F ) Dall-Kirkham telescope Johnson V band filter Standard Johnson UBVRI filter CCD camera Princeton Instruments 512x512 electrical cooling camera sodium column density (CNa) sodium Lidar CSSC sodium Lidar sodium layer centroid height (L) laser power (P ) power meter ThorLabs PM100D with S120C sensor atmosphere transmission (Ta) auxiliary telescope 25cm telescope Table 1: Measurement equipments for measuring parameters in the Lidar equation Fig. 1: (Left) Layout plan of the LGS coupling efficiency measurement facility for Xing Long test campaign. Similar layout was applied also for previous field tests. (Right upper) Layout plan of the beam transfer optics for the LGS laser. (Right lower) Actual layout of the laser bench. The red circle marks where asynchronous laser parameter measurement equipments can be switched in/out. 2 SITES PREPARATION AND EQUIPMENTS SETUP As mentioned in equation1, several parameters has to be measured simultaneously to determine the cou- pling efficiency of the laser. These parameters and related measurement equipments used during our field tests are listed in table1. The left pane of figure1 shows our setup used during 2015 Xing Long test campaign (Feng(2015), Feng et al.(in preparation)), similar layouts were adopted in our previous tests with minor changes to accomodate space constraints (Gao Mei Gu 2013 test, Jin et al.(2014), Gao Mei Gu 2014 test, Jin et al. (2015), Canada Vancouver UBC 2013 test, Otarola et al.(accepted)). The LGS laser and the sodium Lidar laser are located in a modified clean room in the laboratory building of the site. Two hatches are installed on the laboratory ceiling right above the Laser Launching Telescope (LLT) of the LGS laser and the zenith pointing fold mirror of the Lidar laser respectively for launching lasers to the zenith. A make-shift cottage is built 20 meters away from laser launching points. The choice of its location is limited by surrounding 4 L. Feng et al. buildings and terrain, but decided not to be too far away from the launching points to minimize LGS spot elongation and synchronous delay for Lidar. The cottage also has two hatches installed on its roof. The 32cm LGS photometry telescope and a 50cm Dobsonian telescope for Lidar system are set up under these hatches respectively. A 25cm auxiliary telescope provided by Xing Long site is used routinely during test nights to monitor the atmospheric transmission. The scheme for LGS laser beam transfer optics are shown in the right pane of1. The 589.159nm sodium laser comes out from the port side of the package. A combination of the half-wave plate and the thin-film polarizer acts as power attenuator for adjusting projected laser power. An Electro Optics Modulator (EOM) is used to generate sodium D2b line sidebands from the original D2a line for D2b repumping technique which will bring enhancement for photon flux return (Kibblewhite(2009)). A Quarter-Wave Plate (QWP) is added after the EOM to adjust the polarization of the output laser beam. A pair of lens is inserted to adjust the laser beam width to fully fill the input aperture of the LLT. Large space is intentionally left between the 1st and 2nd fold mirrors after the beam expander.
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